Thick haze engulfed Delhi today as the air quality in the national capital remained 'severe' for the third consecutive day, with mild rain, further adding to pollution woes, authorities said.

The Central Pollution Control Board or CPCB recorded an overall air quality index (AQI) of 413. An AQI between 201 and 300 is considered 'poor', 301 and 400 'very poor' and 401 and 500 'severe'.

The AQI on Monday was recorded at 412 and on Tuesday it was at 415. According to experts, even healthy people find it hard to breathe when the air quality level is at 'severe' and doctors advise physical activity to be kept at a minimum.

Neighbouring Ghaziabad and Noida also recorded 'severe' air quality. Ghaziabad's air quality was the worst at an AQI of 429, the CPCB data showed. Faridabad recorded 'very poor' air quality.

Twenty-nine areas in Delhi recorded 'severe' air quality and in eight areas it was at 'very poor', the CPCB added.

The overall PM2.5 level - fine particulate matter in the air with a diameter of less than 2.5 micrometre - was recorded at 257 and the PM10 level at 423.

The government-run System of Air Quality and Weather Forecasting (SAFAR) said that the meteorological conditions leading to insufficient rain led to deterioration of the air quality.

"There is a fall in temperature and good amount of moisture is present in the air. Moist air is passing over Delhi's cool surface leading to fog formation and calm winds are not allowing pollutants to disperse," the SAFAR said.

According to the IMD, the air quality is likely to remain under 'severe' category for next two days. Situation is likely to improve after that when wind speed picks up. The relative humidity was recorded on Wednesday at 98 per cent, the weather office said.